Stirling A. Colgate

Inside the supernova: A powerful convective engine

Condensed Abstract: We present an extensive study of the inception of supernova explosions by following the evolution of the cores of two massive stars (15 Msun and 25 Msun) in two dimensions. Our calculations begin at the onset of core collapse and stop several 100 ms after the bounce, at which time successful explosions of the appropriate magnitude have been obtained. (...) Guided by our numerical results, we have developed a paradigm for the supernova explosion mechanism. We view a supernova as an open cycle thermodynamic engine in which a reservoir of low-entropy matter (the envelope) is thermally coupled and physically connected to a hot bath (the protoneutron star) by a neutrino flux, and by hydrodynamic instabilities. (...) In essence, a Carnot cycle is established in which convection allows out-of-equilibrium heat transfer mediated by neutrinos to drive low entropy matter to higher entropy and therefore extracts mechanical energy from the heat generated by gravitational collapse. We argue that supernova explosions are nearly guaranteed and self-regulated by the high efficiency of the thermodynamic engine. (...) Convection continues to accumulate energy exterior to the neutron star until a successful explosion has occurred. At this time, the envelope is expelled and therefore uncoupled from the heat source (the neutron star) and the energy input ceases. This paradigm does not invoke new or modified physics over previous treatments, but relies on compellingly straightforward thermodynamic arguments. It provides a robust and self-regulated explosion mechanism to power supernovae which is effective under a wide range of physical parameters.

ROSSBY WAVE INSTABILITY OF THIN ACCRETION DISKS - II. DETAILED LINEAR THEORY

In an earlier work we identi—ed a global, nonaxisymmetric instability associated with the presence of an extreme in the radial pro—le of the key function L(r) 4 (&)/i2)S2@! in a thin, inviscid, nonmagnetized accretion disk. Here &(r) is the surface mass density of the disk, )(r) is the angular rotation rate, S(r )i s the speci—c entropy, ! is the adiabatic index, and i(r) is the radial epicyclic frequency. The dispersion relation of the instability was shown to be similar to that of Rossby waves in planetary atmospheres. In this paper, we present the detailed linear theory of this Rossby wave instability and show that it exists for a wider range of conditions, speci—cally, for the case where there is a ii jump ˇˇ over some range of r in &(r) or in the pressure P(r). We elucidate the physical mechanism of this instability and its dependence on various parameters, including the magnitude of the ii bump ˇˇ or ii jump,ˇˇ the azimuthal mode number, and the sound speed in the disk. We —nd a large parameter range where the disk is stable to axisym- metric perturbations but unstable to the nonaxisymmetric Rossby waves. We —nd that growth rates of the Rossby wave instability can be high, for relative small jumps or bumps. We discuss possible D0.2) K conditions which can lead to this instability and the consequences of the instability. Subject headings: accretion, accretion diskshydrodynamicsinstabilitieswaves

What Can the Accretion-induced Collapse of White Dwarfs Really Explain?

The accretion-induced collapse (AIC) of a white dwarf into a neutron star has been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of problematic neutron star populations and specific binary systems. The ejecta from this collapse has also been claimed as a source of r-process nucleosynthesis. So far, most AIC studies have focused on determining the event rates from binary evolution models and less attention has been directed toward understanding the collapse itself. However, the collapse of a white dwarf into a neutron star is followed by the ejection of rare neutron-rich isotopes. The observed abundance of these chemical elements may set a more reliable limit on the rate at which AICs have taken place over the history of the Galaxy. In this paper, we present a thorough study of the collapse of a massive white dwarf in one- and two-dimensions and determine the amount and composition of the ejected material. We discuss the importance of the input physics (equation of state, neutrino transport, rotation) in determining these quantities. These simulations affirm that AICs are too baryon rich to produce gamma-ray bursts and do not eject enough nickel to explain Type Ia supernovae (with the possible exception of a small subclass of extremely low-luminosity Type Ias). Although nucleosynthesis constraints limit the number of neutron stars formed via AICs to 0.1% of the total Galactic neutron star population, AICs remain a viable scenario for forming systems of neutron stars that are difficult to explain with Type II core-collapse supernovae.

Magnetic Energy of the Intergalactic Medium from Galactic Black Holes

We present a quantitative analysis of two radio source samples having opposite extremes of ambient gas density that leads to important new conclusions about the magnetic energy in the intergalactic medium (IGM). We analyze here (1) a new, large sample of well-imaged giant extragalactic radio sources that are found in rarefied IGM environments and (2) at the other extreme, radio galaxies situated in the densest known IGM environments, within 150 kpc of rich cluster cores. We find that sources in the former sample contain magnetic energies EB ~ 1060-1061 ergs and could be viewed as important calorimeters of the minimum energy a black hole (BH) accretion disk system injects into the IGM. In contrast to the radiation energy released by BH accretion, most of the magnetic energy is trapped initially in a volume, up to ~1073 cm3, around the host galaxy. But since these large, megaparsec-scale radio lobes are still overpressured after the active galactic nucleus phase (AGN), their subsequent expansion and diffusion will magnetize a large fraction of the entire IGM. This suggests that the energy stored in intergalactic magnetic fields will have a major, as yet underestimated effect on the evolution of subsequently forming galaxies. Comparison with the second, cluster core-embedded sample shows that the minimum magnetic energy EB can be a strongly variable fraction of the inferred accretion energy Eacc, and that it depends on the ambient IGM environment. Cluster embedded AGNs inject significant energy as PdV work on the thermal ICM gas, and their magnetic energy, even ignoring the contribution from stellar and starburst outflows, is sufficient to account for that recently found beyond the inner cores of galaxy clusters. We discuss the various energy loss processes as these magnetized CR clouds (lobes) undergo their enormous expansion into the IGM. We conclude that the aggregate IGM magnetic energy derived purely from galactic black holes since the first epoch of significant galaxy BH formation is sufficiently large that it will have an important influence on the process of both galaxy and visible structure formation on scales up to ~1 Mpc.

POYNTING JETS FROM ACCRETION DISKS: MAGNETOHYDRODYNAMIC SIMULATIONS

The powerful narrow jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk that acts to extract magnetically the angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic outflows, which have a significant mass flux and which have the energy and angular momentum carried by both the matter and the electromagnetic field, and Poynting outflows, in which the mass flux is negligible and the energy and angular momentum are carried predominantly by the electromagnetic field. Recent simulation studies have focused almost exclusively on hydromagnetic outflows. Here we consider a Keplerian disk initially threaded by a dipole-like poloidal magnetic field. We present the first MHD simulation results establishing that a quasi-stationary collimated Poynting jet arises from the inner part of the disk while a steady uncollimated hydromagnetic outflow arises in the outer part of the disk.

MODELING THE LARGE-SCALE STRUCTURES OF ASTROPHYSICAL JETS IN THE MAGNETICALLY DOMINATED LIMIT

We suggest a new approach that could be used for modeling both the large-scale behavior of astrophysical jets and the magnetically dominated explosions in astrophysics. We describe a method for modeling the injection of magnetic fields and their subsequent evolution in a regime where the free energy is magnetically dominated. The injectedmagneticfields,alongwiththeir associatedcurrents, have bothpoloidalandtoroidalcomponents,andthey are not force free. The dynamic expansion driven by the Lorentz force of the injected fields is studied using threedimensional ideal magnetohydrodynamic simulations. The generic behavior of magnetic field expansion, the interactions with the background medium, and the dependence on various parameters are investigated. Subject headings: galaxies: active — galaxies: jets — magnetic fields — methods: numerical — MHD

Magnetorotational Instability in Liquid Metal Couette Flow

Despite the importance of the magnetorotational instability (MRI) as a fundamental mechanism for angular momentum transport in magnetized accretion disks, it has yet to be demonstrated in the laboratory. A liquid sodium αω dynamo experiment at the New Mexico Institute of Mining and Technology provides an ideal environment to study the MRI in a rotating metal annulus (Couette flow). A local stability analysis is performed as a function of shear, magnetic field strength, magnetic Reynolds number, and turbulent Prandtl number. The latter takes into account the minimum turbulence induced by the formation of an Ekman layer against the rigidly rotating end walls of a cylindrical vessel. Stability conditions are presented, and unstable conditions for the sodium experiment are compared with another proposed MRI experiment with liquid gallium. Because of the relatively large magnetic Reynolds number achievable in the sodium experiment, it should be possible to observe the excitation of the MRI for a wide range of wavenumbers and to further observe the transition to the turbulent state.

MAGNETIC HELIX FORMATION DRIVEN BY KEPLERIAN DISK ROTATION IN AN EXTERNAL PLASMA PRESSURE: THE INITIAL EXPANSION STAGE

We study the evolution of a magnetic arcade that is anchored to an accretion disk and is sheared by the differential rotation of a Keplerian disk. By including an extremely low external plasma pressure at large distances, we obtain a sequence of axisymmetric magnetostatic equilibria and show that there is a fundamental difference between field lines that are affected by the plasma pressure and those that are not (i.e., force free). Force-free fields, while being twisted by the differential rotation of the disk, expand outward at an angle of ~60° away from the rotation axis, consistent with the previous studies. These force-free field lines, however, are enclosed by the outer field lines, which originate from small disk radii and come back to the disk at large radii. These outer fields experience most of the twist, and they are also affected most by the external plasma pressure. At large cylindrical radial distances, magnetic pressure and plasma pressure are comparable so that any further radial expansion of magnetic fields is prevented or slowed down greatly by this pressure. This hindrance to cylindrical radial expansion causes most of the added twist to be distributed on the ascending portion of the field lines, close to the rotation axis. Since these field lines are twisted most, the increasing ratio of the toroidal B component to the poloidal component BR,z eventually results in the collimation of magnetic energy and flux around the rotation axis. We discuss the difficulty with adding a large number of twists within the limitations of the magnetostatic approximation.

Iron Opacity and the Pulsar of SN 1987A

Neutron stars formed in Type II supernovae are likely to be initially obscured by late-time fallback. Although much of the late-time fallback is quickly accreted via neutrino cooling, some material remains on the neutron star, forming an atmosphere that slowly accretes through photon emission. In this paper, we derive structure equations of the fallback atmosphere and present results of one-dimensional simulations of that fallback. The atmosphere remaining after neutrino cooling (Lν) becomes unimportant (LνLEdd, e−, the Compton Eddington limit) is only a fraction of the total mass accreted (10-8 Macc=10−9 M☉). Recombined iron dominates the opacity in the outer regions, leading to an opacity 103-104 times higher than that of electron scattering alone. The resultant photon emission of the remnant atmosphere is limited to 10−3LEdd, e−. The late-time evolution of this system leads to the formation of a photon-driven wind from the accretion of the inner portion of the atmosphere, leaving, for most cases, a bare neutron star on timescales shorter than 1 yr. The degenerate remnant of 1987A may not be a black hole. Instead, the fallback material may have already accreted or blown off in the accretion-driven wind. If the neutron star has either a low magnetic field or a low rotational spin frequency, we would not expect to see the neutron star remnant of 1987A.

Giant Radio Galaxies and Cosmic-Ray Acceleration

Giant radio galaxies (GRGs) are prime and unique laboratories for constraining the plasma processes that accelerate relativistic electrons within large intergalactic volumes. The evidence for short radiative loss times rules out certain scenarios for energy transport within their very large dimensions. This, combined with their high energy content, large ordered magnetic field structures, the absence of strong large-scale shocks, and very low upper limits on their internal thermal plasma densities, points to a direct and efficient conversion of force-free magnetic field to particle energy. This is underlined by the evidence in GRGs that their internal Alfven speeds are higher than the lobe expansion speeds. We discuss these constraints in the context of models in which the central black hole energy is initially extracted as electromagnetic Poynting flux that injects large amounts of magnetic flux into the lobes. Recent advances in the theory of collisionless magnetic reconnection make this a favored mechanism to explain the particle acceleration in these systems. The energy reservoir is likely to be force-free fields, which is independently consistent with recent models of initial electromagnetic energy transfer from the parent galaxys supermassive black hole. Such a scenario has wide-ranging astrophysical consequences: it implies that space-distributed magnetic reconnection or some other highly efficient field-to-particle energy conversion process likely dominates in all extended extragalactic radio sources.